Constant Altitude Plan Position Indicator (CAPPI) is a form of data presentation in weather radars. For CAPPI scanning, a horizontal slice is taken through the radar volume scan data at a constant altitude above the earth surface. The radar volume scan data is extracted from full 3-D scan data, and is converted to a 2-D format for presentation in polar coordinates on a computer display, paper printout, or any other two-dimensional surface.
CAPPI is a variant of the more general Plan Position Indicator (PPI) that displays weather parameters along radials from the radar as function of an azimuth scan angle. A radar antenna transmits and receives pulses at different elevation angles φ and at different azimuth angles θ by both performing a rotating scan operation in the azimuth and by varying the elevation angle. PPI data is generated and recorded by scanning a beam circularly at a constant elevation angle. A volume scan consists of multiple constant-elevation azimuth scans. PPI volume scan data at multiple elevation angles is used to produce CAPPI.
Near the radar site there is often ground clutter, which may interfere with obtaining a clean display of weather. In the beam position(s) with low elevation angle(s), clutter is often so strong that filtering the ground clutter also removes weather signals resulting in gaps in the weather display.
In PPI scanning, the radar beam may overshoot precipitation altitudes, for a part of the radial, and thereby not detect any precipitation at the corresponding ranges (i.e. distances from the radar).
Weather radar systems often deliberately degrade the time resolution for observation in order to improve signal quality and also to reduce the data handling specifications over long observation periods. The PPI volume scanning mode also degrades the spatial resolution by skipping certain elevation angles to reduce the time for scanning the region around the radar.
The maximum elevation of scan may be limited to a value less than 90° (i.e. vertical pointing), leaving a conical ‘blind zone’ over the radar location. This causes a circular hole to appear in the CAPPI, the hole being larger at higher altitudes.